mm/mempolicy: fix a race between offset_il_node and mpol_rebind_task

Servers happened below panic:

  Kernel version:5.4.56
  BUG: unable to handle page fault for address: 0000000000002c48
  RIP: 0010:__next_zones_zonelist+0x1d/0x40
  Call Trace:
    __alloc_pages_nodemask+0x277/0x310
    alloc_page_interleave+0x13/0x70
    handle_mm_fault+0xf99/0x1390
    __do_page_fault+0x288/0x500
    do_page_fault+0x30/0x110
    page_fault+0x3e/0x50

The reason for the panic is that MAX_NUMNODES is passed in the third
parameter in __alloc_pages_nodemask(preferred_nid).  So access to
zonelist->zoneref->zone_idx in __next_zones_zonelist will cause a panic.

In offset_il_node(), first_node() returns nid from pol->v.nodes, after
this other threads may chang pol->v.nodes before next_node().  This race
condition will let next_node return MAX_NUMNODES.  So put pol->nodes in
a local variable.

The race condition is between offset_il_node and cpuset_change_task_nodemask:

  CPU0:                                     CPU1:
  alloc_pages_vma()
    interleave_nid(pol,)
      offset_il_node(pol,)
        first_node(pol->v.nodes)            cpuset_change_task_nodemask
                        //nodes==0xc          mpol_rebind_task
                                                mpol_rebind_policy
                                                  mpol_rebind_nodemask(pol,nodes)
                        //nodes==0x3
        next_node(nid, pol->v.nodes)//return MAX_NUMNODES

Link: https://lkml.kernel.org/r/20210906034658.48721-1-yanghui.def@bytedance.com
Signed-off-by: yanghui <yanghui.def@bytedance.com>
Reviewed-by: Muchun Song <songmuchun@bytedance.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 5e90b3f..b44f547 100644 (file)
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -1979,17 +1979,26 @@ unsigned int mempolicy_slab_node(void)
  */
 static unsigned offset_il_node(struct mempolicy *pol, unsigned long n)
 {
-       unsigned nnodes = nodes_weight(pol->nodes);
-       unsigned target;
+       nodemask_t nodemask = pol->nodes;
+       unsigned int target, nnodes;
        int i;
        int nid;
+       /*
+        * The barrier will stabilize the nodemask in a register or on
+        * the stack so that it will stop changing under the code.
+        *
+        * Between first_node() and next_node(), pol->nodes could be changed
+        * by other threads. So we put pol->nodes in a local stack.
+        */
+       barrier();
 
+       nnodes = nodes_weight(nodemask);
        if (!nnodes)
                return numa_node_id();
        target = (unsigned int)n % nnodes;
-       nid = first_node(pol->nodes);
+       nid = first_node(nodemask);
        for (i = 0; i < target; i++)
-               nid = next_node(nid, pol->nodes);
+               nid = next_node(nid, nodemask);
        return nid;
 }